THE death toll from the Asian tidal waves - currently 50,000 and rising by the hour - could double because of the risk of disease, the World Health Organisation warned last night.

Meanwhile, aid officials described the tsunami horror as one of the world's worst natural disasters as the United Nations said the cost of rebuilding devastated areas would run to billions of dollars.

Dr David Nabarro, a World Health Organisation expert based in Geneva, said: "There is certainly a chance that we could have as many dying from communicable diseases as from the tsunami."

He said the main threat to life was communicable diseases associated with a lack of clean water and sanitation.

"The initial terror associated with the tsunamis and the earthquake itself may be dwarfed by the longer-term suffering of the affected communities," he warned.

Scores of decomposed bodies were still being pulled from coastlines last night, while aid agencies attempted to distribute medicine, food and water in the largest relief operation that has ever been mounted.

Health experts said imminent outbreaks of malaria and cholera could prove as deadly as the effect of the undersea earthquake, which registered 9.0 on the Richter scale - the world's strongest in 40 years.

Yvette Stevens, UN emergency relief co-ordinator, said last night that the disaster was unprecedented and would be the costliest ever.

Killer waves, some 20ft high, were created by the quake off the west coast of the Indonesian island of Sumatra, on Sunday at 0059 GMT.

Their impact was felt on the coast of Somalia in eastern Africa - almost 3,000 miles from the earthquake's epicentre.

The Thai government was accused of playing down warnings of the disaster in an attempt to protect the country's lucrative tourist industry.

Officers from the country's meteorological department said information sent to tourist resorts had deliberately underestimated the threat.

The highest death tolls were in Indonesia and Sri Lanka - which suffered about 20,000 each - followed by thousands more in India and Thailand, and dozens in Somalia, Myanmar (Burma), Malaysia, and the Maldives.

There were also deaths in Tanzania, the Seychelles, Bangladesh and Kenya.

Emergency workers who reached the northern tip of Sumatra island, closest to the quake epicentre, found that 10,000 people had been killed in the town of Meulaboh.

Eighteen Britons have so far been confirmed among the fatalities - 12 in Thailand, three in Sri Lanka and three in the Maldives.

A six-year-old boy from St Ives, in Cornwall, who was on holiday with his family in Thailand, was confirmed dead.

The unnamed boy's eight year-old brother was missing presumed dead, along with his mother's 44-year-old boyfriend, who was originally from Essex.

The mother, who is 37, was apparently uninjured.

A British embassy official in Thailand said: "We are aware that there are a very large number of bodies as yet unidentified, in particular in Ko Lak.

"There has to be a danger that several, if not more, of the dead will be British."

In Sri Lanka, British embassy staff said three teams were scouring resorts for missing Britons, with scores feared dead.

Across the region, bodies were decomposing along roads, beaches and in towns, as hospitals and morgues became choked with corpses.

People in Sri Lanka began digging graves with their bare hands, anxious to bury their dead.

Aid workers have been warned that flooding has uprooted landmines in the war-torn country, leaving them at risk as they help survivors.

In Indonesia, people have been forced to loot stores as food and water supplies run perilously low.

Across Asia, millions remain homeless and thousands are missing, with fears that hundreds of tourists, fishermen and villagers were simply swept out to sea and to their deaths.